112 



THE IRRIGATION AGE. 



impossible to know the exact ability of a 

 stream for supplying water unless it has 

 been gauged or measured for a number of 

 years during the season of minimum flow. 



METHODS OF MEASURING. 



There are a number of methods in use 

 for determining the flow of a stream of 

 water confined in a fairly well- defined 

 channel of its own. Sometimes the result is 

 attained by determining the mean velocity 

 of the flow and measuring the area of cross- 

 section occupied by the flowing water, 

 while at other times it may be more con- 

 venient to erect a measuring weir, and by 

 the use of a weir formula determine the 

 rate of flow. 



If the former of these methods is used, 

 the velocity may be determined by a cur- 

 rent meter, or the slope and cross- sec- 

 tion may be measured and some suitable 

 formula for the flow of water in open chan- 

 nels employed for calculating the result. 



Circumstances must govern an engineer 

 almost wholly as to which method he 

 should employ. In the case of a stream, 

 which is neither too large nor too rapid to 

 prevent the construction of a measuring 

 weir across it at a reasonable expense, this 

 is the best method to adopt, as the result 

 obtained is invariably more accurate than 

 by any other method. On the other hand, 

 when a stream is either too large or swift, 

 or cannot be easily controlled over a weir 

 for some other reason, it is better to de- 

 termine the velocity of the water and the 

 area of cross-section it occupies, and let 

 the result depend on these determinations, 

 rather than to attempt the more expensive 

 expedient of constructing a measuring 

 weir. 



The details and formulae for measuring 

 the flow of streams will be fully discussed 

 in succeeding chapters devoted to the flow 

 of water in open channels and over measur- 

 ing weirs. 



UNDERFLOW OF STREAMS. 



All the water supply of a river or creek 

 is not always visible on the surface of the 

 ground constituting the bed of the stream. 

 Sometimes a large proportion of the water 

 flows under the surface of the ground en- 

 tirely and is not visible, giving no ocular 

 evidence of the existence of more than that 

 which flows in the channel of the stream. 

 Again, the entire flow of a stream may ex- 

 ist under the surface of the bed and no 

 water be visible at all, except during pe- 



riods of storms and freshets. Such streams 

 are usually called " dry creeks, " the fact 

 that no water is in sight being taken as 

 conclusive evidence that the water-courses 

 in question possess no permanent and 

 regular water supply. 



But in regions where water is scarce, 

 either for domestic or irrigation purposes, 

 explorations have been made below the 

 surface, which have demonstrated that an 

 underflow exists under the channels of 

 most streams whether there is any surface 

 flow or not. Such underflows have not 

 only been often discovered, but in many 

 cases have been fully developed and are 

 now being used as a permanent source of 

 water supply for domestic and irrigation 

 water systems. 



The character and extent of the under- 

 flow of streams is in a great measure 

 controlled by the same conditions which 

 govern the surface flow, and must be inves- 

 tigated and analyzed in much the same 

 manner. The same information in regard 

 to rainfall, watershed, etc. , which has al- 

 ready been discussed in the preceding 

 articles in connection with the surface 

 flow of streams, must also be collected and 

 used in investigating the underflow of a 

 stream as a source of water supply for ir- 

 rigation. In addition there are many 

 other subjects to be considered which will 

 be discussed in the succeeding articles. 



DEFINITION OF UNDERFLOW. 



The underflow of a stream is water de- 

 rived from rainfall descending upon the 

 watershed tributary to the stream, which 

 sinks into the ground until intercepted by 

 the first impervious layer encountered and 

 then percolates through the coarse mate- 

 rial composing the bed of the stream in 

 practically the same direction as the sur- 

 face drainage. The above definition cov- 

 ers all the points which distinguish an un- 

 derflow and enables us to identify it as 

 such. 



It is clearly distinguished from artesian 

 water, which also originates from rainfall 

 entering the ground before it reaches the 

 channel of a stream, but is confined be- 

 tween two or more impervious formations, 

 and is usually found at a considerable 

 distance from the point where it first 

 sinks, in the fact of its existence on top of 

 the first impervious stratum encountered 

 after penetrating the loose material of the 

 channel, and in being devoid of pressure 



